Unfortunately, that tutorial doesn’t apply to your theme.
You could try adding the following to your Site Header in Settings > Code Injection. Because this change could affect lots of different pages on your site, you’ll want to test other pages to make sure nothing looks askew.
In the example below, I’ve increased the width by 200 px. However, you can change the 920
value below up or down to suit your needs.
<style>
.gh-canvas {
grid-template-columns: [full-start] minmax(4vmin, auto) [wide-start] minmax(
auto,
240px
) [main-start] min(920px, calc(100% - 8vw)) [main-end] minmax(auto, 240px) [wide-end] minmax(
4vmin,
auto
) [full-end];
}
</style>